The invention relates to a semiconductor diode laser with a monitor diode, comprising a semiconductor body with a substrate region and a superimposed semiconductor layer structure comprising an active layer and a pn junction with which, provided there is a sufficiently strong current in the forward bias direction of the pn junction, coherent electromagnetic radiation can be generated in a strip-shaped active region of the active layer situated within a resonance cavity, which resonance cavity is bounded by two end faces which are substantially perpendicular to the active region, the radiation emerging through at least one of the end faces, while the monitor diode is formed by a portion of the semiconductor body which is separated from the diode laser by a groove which extends through the layer structure down to at least the substrate region and of which one of the walls forms an end face of the diode laser.
Such a semiconductor diode laser is described in U.S. Pat. No. 4,653,058.
Semiconductor laser diodes with a layer structure comprising a strip-shaped active region with a pn junction which produces laser radiation in a direction parallel to the pn junction when biased in the forward direction are known in various embodiments.
A monitor diode is generally used for keeping the intensity of the generated laser radiation constant or for varying it in a desired manner. This monitor diode, which is usually reverse-biased, is so positioned that at least part of the laser radiation is incident on it and is converted into an electrical signal. This signal is subsequently fed back to the laser supply and used for controlling the current through the diode laser, and thus the radiation intensity supplied by the laser, in the desired manner.
The monitor diode is usually positioned at the rear of the laser diode then, i.e. at the side facing away from the side where the useful laser radiation emerges, which side is called the front hereinafter. Sufficient radiation then issues through the rear mirror surface, which is partly reflecting, in order to control the monitor diode.
If the rear mirror is totally reflecting, however, the monitor diode cannot be positioned at the rear. Monitoring of the energy radiated through the rear is also either impossible or insufficient for achieving the set aim if during the laser life, or in the operating state, the intensity ratio between the "front" and the "rear" radiation changes. This may occur in particular in the case of re-tuning of tunable laser diodes, for example of the DBR (Distributed Bragg Reflection) type. In this case the monitor diode must be positioned in the light path of the main radiation beam emitted at the front. The monitor diode should also be provided at the front especially when the aim is to suppress noise in the useful laser radiation.
Such an arrangement of the monitor diode, however, has major disadvantages. The monitor diode may absorb a substantial portion of the radiation. Compensation of this through an increase of the current through the laser is not always possible without exceeding the maximum permissible laser current. In addition, the coupling of the laser radiation in the fibre optics may be seriously hampered or even become impossible in this monitor diode arrangement.